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Marcel doesn't use MLEs - if a player hasn't played at all (or enough) in MLB, the projection is just league average. If you want Marcels + MLEs, then you're talking about Oliver.
The point is that the system uses only regression to the mean and an aging curve. As you say, there's nothing complicated about it, and it's not really intended to be a 'real' projection system - it's a useful benchmark to grade the 'real' projection systems like PECOTA or CHONE. Problematically, it keeps beating many of them. CHONE seems to do well v. Marcel, but the little monkey's done very well vs. PECOTA.